1. Field of the Invention
The invention relates to a seismic acquisition system provided with decentralized processing. More particularly, the invention relates to a data acquisition system including at least one acquisition unit suited for carrying out various processings of data acquired locally and for reducing to a large extent the volume of data transferred between the units and a central control and recording station or a portable control box.
Such a system may for example be used in the field of seismic prospecting where a large number of seismic receivers (each made up of one or several elementary pickups combined to form a "string" and delivering a signal called a "trace") has to be set in a zone to be explored, these receivers being placed in contact with a subsurface formation, and for achieving emission-reception cycles with the transmission in the ground of seismic signals through the controlled triggering of a seismic source causing the reception of the signals reflected by the subsoil discontinuities and the acquisition of the signals. Such an acquisition is achieved for example by means of a plurality of acquisition boxes distributed in the field, each digitizing and storing the signals received in reference to one or several pickups and, by an order from a central station, transmitting them sequentially thereto by cable or by radio.
2. Description of the Prior Art
Various acquisition systems are described for example in French Patents 2,511,722, 2,538,561, 2,599,533, 2,627,652 or in the French patent application EN.90/16,443 corresponding respectively to U.S. Pat. Nos. 4,583,206, 4,979,152, 4,815,044, 4,908,655, 5,276,655 filed by the applicant.
The current trend consists in using increasingly complex acquisition systems with a great number of receivers sometimes distributed over great distances and of acquisition boxes. The volume of data acquired is often very high. The number of seismic traces to be centralized may exceed 1000 in many cases. Transferring them to the central station, preferably at the end of each period of reception of the seismic signals, is a complicated task. The central station must be powerful enough to manage in real time the centralization thereof on the one hand and, on the other hand, to perform in real time a certain number of preprocessings: trace correlation, combination of traces coming from multiples pickups such as triaxial geophones, etc.
Transmission of a great quantity of data is another cause of difficulties. If radio links are used for this centralization, as it is often the case in practice, the restrictions which are often imposed by local regulations concerning the use and the availability of transmission frequencies have to be taken into account. The alternative consisting in transmitting the data to be centralized by means of transmission lines also involves drawbacks as the cables capable of transmitting high rates are complex and costly. It is therefore important to reduce as much as possible the volume of data to be transmitted in order to facilitate its transmission as well as its processing after the transmission thereof.
Before the acquisition operations which will be performed with the system of receivers and of acquisition boxes start or, if need be, during these operations, it is also usual to test the operation of each receiver and/or of each associated acquisition box to determine if operation thereof is trouble free. The pickups (geophones, hydrophones) are considered to be the most vulnerable parts of seismic acquisition chains. It is necessary to check whether the static and dynamic electromechanical parameters required for to each receiver (such as electric resistance, sensitivity, natural resonance frequency or damping factor) are in accordance with the specifications and whether the pickups constituting the receivers are properly coupled to the ground, knowing that an incorrect positioning leads to a significant change in the damping factor and in the sensitivity thereof.
It is well-known to test pickups such as moving-coil geophones by applying thereto a current step and by recording their response curve to this current step. Such a testing method, described for example in the assignee's French Patent 2,613,496 corresponding to U.S. Pat. No. 4,862,425 is used to check the operation of geophones located in well sondes to determine if they are trouble free. Switch devices are placed between each geophone to be tested and the input of an electronic acquisition chain for amplifying, digitizing and storing the signals delivered by the pickup. Application of calibrated current steps, then connection of the excited pickup to the associated acquisition chain and remote transmission in a coded form allow data relating to the pickup itself and to the associated acquisition and transmission chain to be obtained.
The testing of pickups is based on the analysis of their response curve at a given electric current step. This curve generally includes as illustrated in (FIG. 7) a first amplitude peak U.sub.M of "width" T, and a second amplitude peak U.sub.m. It is known how to calculate approximately the damping factor by combining these three characteristics.
This method is not very accurate since the few measurements which are combined depend much on the acoustic and electronic noise, and it is ineffective if, because of too fast a damping, the response curve does not have a second peak.